1. Field of the Invention
This invention relates to an image forming apparatus which is able to record high quality images including halftone images, based on image data such as that input from a host computer, an image reader, a controller, a communication line etc.
2. Description of the Related Art
Recently, printing apparatuses using an electrophotographic method, such as laser printers and the like, have widely been used as computer output apparatuses. Such printing apparatuses have advantages of high picture quality, low noise and the like, and have rapidly spread particularly in the field of desk-top publishing from the viewpoint of high picture quality.
Demand for high picture quality in halftone-image outputs as well as in line-image outputs of such printers has been more and more increasing.
A dither method is widely used when a halftone image is output. In the dither method, each density in a halftone image is represented by a dither matrix of predetermined size. Therefore, the dither method has the problem that if the dither matrix is made large, resolution is decreased while density gradation is excellent, and if the dither matrix is made small, density gradation in decreased while resolution is increased.
As a method for solving the problem, there is a pulse-width modulation method which has recently been practiced in laser printers. According to this method, since many gradation steps can be obtained with a small area, resolution and gradation can be compatible. Circuitry shown in FIG. 9 is a schematic block diagram of a pulse-width modulation method. As shown in FIG. 9, the circuitry includes a pattern signal generation circuit 301, a D/A converter 302 and a comparison circuit 303. An input digital image signal converted into an analog signal by the D/A converter is compared with a pattern signal, and printing is performed when the image signal is larger than the pattern signal, as shown for example in FIG. 10(a).
In the above-described pulse-width modulation method, however, when low-density image data are input, the pulse width becomes very narrow, and the output signal cannot be sufficiently developed. As a result, an image is obtained in which highlight portions become too white.
Referring, for example, to FIG. 10(b), a phenomenon occurs in which printing is performed with a pulse width "b", while printing is not performed with a pulse width "a".
In the above-described printing apparatuses, a print image is constituted by a plurality of picture elements (pixels). The current density of picture elements is 240 dpi (dots per inch)-400 dpi, and the size of one picture element is about 60-100 .mu.m. These values very nearly correspond to the resolution (about 80 .mu.m) of a human being having normal visual acuity when viewing an object 30 cm away. Thus, the individual picture elements can be recognized when a curve or an oblique line is represented by a printing apparatus having aa picture-element density of about 300 dpi. Accordingly, demand for printing having higher resolution has increased. Furthermore, demand for halftone representation has also increased.
For such demands for high-resolution printing and halftone printing, the diameter of toner particles currently used is about 10 .mu.m. If it is assumed that toner particles are homogeneous and uniformly cover picture elements, a little more than sixty toner particles adhere for a picture element of 300 dpi. For a picture element of 600 dpi, only a little more than ten toner particles adhere. Actually, toner particles adhere in multiple layers, and so the actual amount of toner particles that adhere is larger than the above-described figures. However, when printing is performed for smaller picture elements having a density of 600 dpi or more, variations in the amount of particles that adhere become a problem, and causes, for example, inconsistent image density. Thus, even though the density is increased, picture quality is not improved. Halftone representation is determined by the amount of toner particles that adhere to a unit area. The size of one picture element when printing at 600 dpi is about 40.times.40 .mu.m, and so the number of adherable toner particles is about a little more than ten. Hence, it is impossible to provide an image having a large gradation range.
In order to solve the above-described problems, toner having a particle size of about 4-7 .mu.m has been developed. The average size of the fine-particle toner is about 1/2 that of conventional toner, and hence 1/4 in area and 1/8 in volume. By using the fine-particle toner in 600-dpi printing, the same quality image can be obtained as when normal toner of about 10 .mu.m is used in 300-dpi printing. However, toner for high definition is higher in cost than conventional toner.
Accordingly, it has become necessary to properly use in a printing apparatus various toners having different particle sizes in accordance with cost. A conventional printing apparatus, however, did not have a method for discriminating toners having different particle sizes. Hence, it was necessary for an operator to set up the use of proper toner.
In the prior art, switching of the processing system in a printer is determined only between a host computer and the printer, and the kind of toner actually used cannot be directly known. Hence, the prior art has the following disadvantages:
(1) When a command for high-density printing is transmitted from the host computer to the printer while low-quality toner is used, printing is performed without changing the toner, and hence a blurred image is obtained.
(2) When a processing system which does not coincide with the kind of toner is assigned from the host computer, it is impossible to notify the computer of the fact. Hence, wasteful data transfer is performed which is thrown away after being transmitted.